An engine system may include exhaust gas recirculation (EGR) to improve engine operation. For example, EGR may reduce engine emissions (e.g., NOx) and improve engine efficiency via reducing engine pumping work. In some examples, external EGR (e.g., EGR routed between an exhaust system and an engine air intake system) may be cooled to further reduce engine NOx and reduce the possibility of engine knock at higher engine loads. Cooling the EGR further lowers cylinder charge temperatures, thereby reducing formation of NOx and auto-ignition of end gases. However, at lower engine loads, cooled EGR may cause combustion stability to degrade and lead to the possibility of misfire, increased engine noise, and increased engine vibration. Thus, it may be desirable to use cooled EGR at middle and higher engine loads and reducing its use at lower engine loads. However, since cooled EGR is supplied to the engine intake system, it may take many engine cycles to clear the cooled EGR from the engine intake system when the engine transitions from higher load conditions to lower load conditions. Consequently, engine emissions and combustion stability may degrade when the engine changes from an operating condition where a higher concentration of cooled EGR is supplied to engine cylinders to a condition where a lower concentration of cooled EGR is supplied to engine cylinders.
The inventors herein have recognized the challenges of operating an engine including cooled EGR and have developed a method for operating an engine including cooled EGR, comprising: supplying cooled EGR to engine cylinders; and increasing a concentration of internal EGR supplied to the engine cylinders in response to a concentration of cooled EGR supplied to the engine cylinders during a reduction in engine load.
By increasing a concentration of internal EGR supplied to engine cylinders during a reduction in engine load, it may be possible to provide a technical result of initiating homogeneous charge compression ignition (HCCI) in engine cylinders so that the engine's tolerance for EGR may be increased. Consequently, instead of decreasing internal EGR to reduce cylinder charge dilution during a reduction in engine torque, the increased internal EGR may facilitate the engine transitioning to a HCCI mode where the higher concentration of cooled external EGR may be consumed by the engine and participate in stable combustion.
The present description may provide several advantages. For example, the approach may reduce the possibility of engine misfires when operating an engine with cooled EGR. Additionally, the approach may reduce engine NOx emissions at lower engine loads. Further, the approach may improve engine fuel economy.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.